1. Technical Field
The present invention relates generally to a system for detecting failure occurring in a control system operating an inductance load. More particularly, the invention relates to a failure detecting system applicable to various control systems such as for a four wheel steering system or an active suspension control system.
2. Background Art
A Japanese Patent First Publication No. 2-18169 discloses a rear wheel steering control system for a vehicle which includes an apparatus for detecting failure occurring in a load driving system.
This control system is operable to determine a rear wheel steering target angle based on a front wheel steered angle and vehicle speed so as to obtain an optimal vehicular dynamic characteristic when turning to steer the rear wheels at a phase angle opposite the direction of front wheels or at the same phase as the front wheels by means of a hydraulic actuator which improves traveling stability and steering responsiveness.
Internally, the above-mentioned control system includes a microcomputer for controlling hydraulic pressure to be applied to the hydraulic actuator and a driving circuit for the latter. The microcomputer calculates a necessary rear wheel steering angle. The driving circuit, in turn, provides current corresponding to the calculated rear wheel steering angle to an electromagnetic valve to apply pressurized hydraulic fluid to the actuator for controlling rear wheel steering. In cases where a wire harness connected between the driving circuit and the electromagnetic valve is broken shutting off the driving current applied to the hydraulic actuator or in case of a short circuit, a maximum current is quickly applied to the hydraulic actuator and the system closes a cutoff valve to block the hydraulic fluid flow to the actuator allowing the remaining hydraulic pressure in the actuator to leak gradually through small clearances in the cutoff valve so as to reduce the hydraulic pressure level therein gradually to prevent sudden vehicular attitude change. Thus, to provide a highly safe control system for rear wheel steering two separate failure detection systems are required one for monitoring operation of the microcomputer and one to monitor the actuator driving circuit.
For monitoring the microcomputer, a clock signal generator is provided which generates clock signals to a signal line in which operation of the microcomputer can be monitored. A watchdog timer monitors the clock signals in a signal current output through the signal line to detect failure such as overload of the microcomputer. For monitoring the driving circuit, the system monitors the current value of a linear driving current flowing in the wire harness to detect failure. However, in cases where a short circuit occurs when driving current is flowing through the wire harness or disconnection of the wire harness occurs at a time when no current is present, the system cannot detect the failure. It will be thus appreciated that the time during which the failure can be detected is limited to a specific period. Additionally, since the system requires two separate failure detecting units for each of the microcomputer and the driving circuit, conventional systems such as described become complex and expensive.
Further, a Japanese Patent First Publication No. 64-63463, which is owned commonly by the assignees of this application, discloses another failure detecting system which includes a clock signal generator interposed between a microcomputer and a driving circuit for a rear wheel steering power cylinder to apply a current with a dither signal to the latter. The system monitors the dither signal in the current output from the driving circuit to detect failure in a system controlling a driven load.
However, because the system detects the failure after the signal is output from the microcomputer, it cannot detect failure in the microcomputer.